Brake drums and brake disks of the type used in many cases in passenger cars and trucks by the motor vehicle industry are subjected to severe dynamic and mechanical stresses produced especially by high thermal shock stress. For instance, when driving motor vehicles down a long gradient with full braking in the mountains, temperatures of up to 800.degree. C. are reached on the brake disks, which may lead with subsequent quick cooling to substantial stresses and thereafter to stress cracks. In practice, therefore, it is advisable to select for the brake disk or brake drum a cast iron material with lamellar graphite, exhibiting a high thermal and temperature conductivity as well as a low thermal expansion coefficient.
Such a high-carbon gray cast iron is described, for instance, in DE 37 40 912 A1. A disadvantage, however, is that this type of cast iron with lamellar graphite exhibits a low modulus of elasticity and a low tensile strength between 100 N/mm.sup.2 and 150 N/mm.sup.2. As a result, such materials are less suited for the area of the hub and the forming of the flange. According to W. Keiner, Designing and Casting15 (1990), No. 4, pages 4-14, the cross-section of heavy brake drums in these areas, especially at the junction to the flange, must be executed with a greater thickness of material and be reinforced by a few millimeters. The brake elements are, as a result, more or less weight-loaded.
Moreover, the hub and the flange are subjected to great dynamic stresses as a result of shocks and impacts, whereby the flange, as a result of thread tapping and the associated wheel bolts, exhibits already a lower strength, resulting, together with the notch effect emanating from the screws, in a reduction of the reverse bending strength.
In order to meet these requirements, it is common to produce brake drums or brake disks of high-grade iron-carbon alloys or other especially suited materials.
In order to reduce the expense in the production of a brake disk composed of various materials, it was suggested in DE 22 63 654 A1, to produce the concentric brake body arranged around the wheel hub in the form of a brake disk or a brake drum as a composite part in a uniform casting. For this purpose, the production of the hub starts with a drop forging of steel or with a cast iron part of spheroidal graphite. In order to improve the bond, a connecting bridge with recesses is provided which is designed as a revolving flange with a thin wall. A complete welding of the metal to the bridge is not envisioned in order to leave a small slot with a sound-absorbing effect between the sheet and the casting metal. On the other hand, a great quenching effect with respect to the cast iron flowing through and the formation of cementite is apparent in a thicker, revolving bridge. Subsequent annealing is therefore required, resulting in an increase of production costs. Furthermore, the smaller cross-section leads to a reduced transmission of power at the junction between the hub and the brake body, causing a substantial limitation in the area of application of the brake disk or the brake drum.
According to the proposed method, this brake element has a revolving parting line located between the brake body and the hub, which leads to heat accumulation during braking and prevents the flow of heat from the brake body towards the hub. The disadvantage is the resulting considerably greater expansion of the brake body in a radial direction which may eventually lead in the course of time to a destruction of the union of the hub with the brake body.
In order to increase the width of application of a brake disk or brake drum produced by composite casting, a durable composite cast union is necessary between the hub and the brake disk or the brake drum or the like, in order to transmit a maximal torque to the brake jaws when running down a long gradient with full braking.
JP A 60 024263 discloses an integrated rotor hub, in which a melting zone structure with good mechanical properties is to be developed at the border area between the rotor part and the hub part. In this production process, the two components are cast in a common mold but with different types of steel. The brake disk consists of cast iron with flaky graphite and with thermally favorable properties, although the hub part consists of a cast iron with spheroidal graphite and high mechanical strength. As a result of the common casting, the separation of the two casting materials is not distinct, and can lead to thermally or mechanically faulty stresses at the components of the brake element. The possible consequences are deformations and cracks presenting a safety risk.